1. Field of the Invention
The present invention relates to a manufacturing method of a pneumatic tire to which an electric resistance countermeasure is applied by providing a conductive layer in a non-conductive tread rubber, and the pneumatic tire.
2. Description of the Related Art
Conventionally, for the purpose of reducing a rolling resistance which has a close relation with a good mileage of a vehicle and improving a braking performance (a wet braking performance) on a wet road surface, there has been known a pneumatic tire in which a tread rubber is blended highly with a silica. However, since the tread rubber mentioned above has a high electric resistance in comparison with the structure blended highly with a carbon black, and obstructs a discharge of a static electricity generated in a vehicle body or the tire to the road surface, there is a problem that a trouble such as a radio noise or the like tends to be generated. Accordingly, there has been developed a pneumatic tire to which an electric resistance countermeasure is applied by providing a conductive layer obtained by blending a carbon black to a non-conductive tread rubber blended with a silica or the like.
For example, in a pneumatic tire described in U.S. Pat. No. 5,518,055, Japanese Unexamined Patent Publication No. 10-81110, Japanese Unexamined Patent Publication No. 10-203114, and Japanese Unexamined Patent Publication No. 2007-8269, a conductive layer is arranged in an interface between a wing rubber or a sidewall rubber and a tread rubber while coating an outer peripheral surface of an end portion of a non-conductive tread rubber, and a static electricity generated in a vehicle body or a tire is discharged to a road surface by bringing an end portion of the conductive layer into contact with a rim or a conductive rubber portion (for example, the wing rubber or the sidewall rubber) which is capable of conducting from the rim.
However, in these tire, since the conductive layer is widely exposed in an outer peripheral surface of the end portion of the tread rubber, a conductive performance is hard to be achieved if the tread rubber wears to some extent. Further, since there is a case that a conductive path is interrupted due to an irregular wear of a shoulder portion, there is a problem that the conductive performance can not be suitably maintained. Besides, since the conductive layer is provided as a sheet shape, a volume of the conductive layer becomes larger than necessary, and since the conductive layer is continuously exposed to the tread surface, there is a problem that it is impossible to sufficiently achieve an improving effect obtained by using the non-conductive tread rubber, that is, an improving effect of a mileage performance and a wet performance in the case that the tread rubber is blended highly with the silica.
U.S. Pat. No. 2,339,546 discloses a pneumatic tire in which one end of a conductive layer provided along a carcass layer is exposed to a tread surface through an inner portion of a tread rubber, and the other end comes into contact with a rim. However, since the conductive layer is formed as a sheet shape which is continuous in a tire circumferential direction, the volume of the conductive layer becomes larger than necessary as mentioned above, and since the conductive layer is continuously exposed to the tread surface, there is a tendency that an improving effect obtained by using the non-conductive tread rubber can not be sufficiently achieved. Further, Japanese Unexamined Patent Publication No. 2007-176437 describes a pneumatic tire in which a conductive layer spirally extends from a rim strip rubber (a clinch rubber) toward a tread surface, however, a conductive path becomes longer than necessary in the case mentioned above.